The present invention relates to a method of operating a solid electrolyte fuel cell using a light hydrocarbon such as natural gas, naphtha or the like as the fuel.
solid electrolyte fuel cells using light hydrocarbons such as natural gases, naphtha, etc. as fuels and air as the oxidant are well known, and a typical cell configuration for such solid electrolyte fuel cells is disclosed in Japanese Kokai Paten Publication No. 57-130381.
This configuration comprises a tubular porous support and formed serially thereon an inner tubular electrode (air electrode), an electrolyte and an outer electrode (fuel electrode), with air being passed inwardly of said porous support and the fuel over the exterior of the outer electrode.
The prior art fuel cell using the above cell configuration is assembled and operated under the following conditions.
1) The fuel utilization rate is increased (80.about.85%) and the fuel is not recycled.
2) The air as such is used as the oxidant at the air utilization rate of about 25%, with the necessary cooling being effected on the air (internal) side.
However, in this conventional solid electrolyte fuel cell configuration, the standard output density per unit area is 150 mW/cm.sup.2 and the electrical output per unit cell is approximately 20 W. This means that about 50 cells are required for the generation of 1 KW and the consequent high cell cost has been a serious drawback in the commercial implementation of a solid electrolyte fuel cell.
The following may be mentioned as reasons why the output density is not high (or cannot be high).
(1) Since the diffusion resistance generated as the air (oxygen) is passed through the tubular porous support and porous air electrode causes a rapid increase in the internal resistance at a current density of about 250 mA/cm.sup.2 or more, the output density cannot be increased.
(2) Since the solid electrolyte is an oxygen ion conductor and the theoretical electromotive voltage drops with an increasing fuel utilization rate, the output density cannot be increased.
In order to attenuate the diffusion resistance mentioned under (1) above, it might be contemplated to increase the degree of porosity of the air electrode. However, if this be done, the ohmic resistance of the air electrode will be increased, and even in a solid electrolyte fuel cell employing the cell configuration taught by Japanese Kokai Patent Publication No. 57-130381, this resistance will be as large as 65% of the total resistance of the cell as a whole. (This value is approximately 8-fold as large as the resistance calculated on the assumption that the air electrode is a dense member.) Therefore, any further improvement can hardly be accomplished by increasing the porosity of the air electrode.
In addition, the prior art solid electrolyte fuel cell has the following drawbacks.
(3) Because of the low theoretical electromotive voltage at the exit, it is ensured that no mixing of the fuel will take place but this entails a large variation in the temperature distribution of the generating chamber.
(4) Because the fuel is not recycled, approximately 15 to 20% of the fuel is wasted.